A broad beam is normally required during the search mode to provide area coverage and to satisfy time on target needs for acquisition purposes. Use of a broad beam, however, reduces the potential operating range by diluting the energy over a large spatial-angular region. This loss in range potential is especially severe for propagation through glossy mediums such as seawater.
In presently known air to undersea communication systems, the ratio of laser beam divergent angle between search and track modes is around four. Extremes in the choice of beam angle for operation at different altitudes is also typically on the order of eight.
The beam power density, S.sub.r, at the receivers can be expressed as; ##EQU1##
Equation (1) shows that P is proportional to (.phi.h).sup.2 for constant S.sub.r and kd. For a given height, h, reducing .phi. by a factor of 2 is equivalent to increasing P by a factor of 4. More generally the effect upon P of a change in .phi. from .phi..sub.1 to .phi..sub.2 is expressed by EQU dB = 20 log .phi..sub.1 /.phi..sub.2 ( 2)
where dB enhances P for .phi..sub.1 &gt;.phi..sub.2.
One attenuation length, AL, is equal to 4.34 dB. Equation (2) is therefore expressable as a change in depth, .DELTA.d, ##EQU2##
Equation (2) or (3), as applied to three types of water, is shown in the graph of FIG. 1.